There are a number of applications of electromagnetic fields which are used in a submarine environment. Marine electromagnetic operation is an important tool for locating off-shore hydrocarbon reserves or for determining the content of a previously located reservoir. It may also be used for monitoring hydrocarbon production for reservoir management once a well bore has been drilled and oil is being extracted. One known procedure involves the use of an electromagnetic source and one or more receivers as described in the present applicants' WO01/57555. Electromagnetic energy generated by the source propagates in all directions and that which propagates downwards react with subsurface layers and the signal returns to the seafloor where it is detected by the receiver. At short offsets there will be direct waves and reflected waves dominating the signal and at longer offsets the signal can be dominated by partially guided waves.
A typical CSEM data set is characterized by large attenuation of the propagated signal. This attenuation is dominated by the distance the signal has propagated. Therefore, at larger offsets or where the target is buried more deeply, the size of the signal received may be small and it may be difficult to interpret the results to determine whether or not a reservoir of interest is present. Similarly, when monitoring an existing reservoir to determine the relative content of oil compared to water, the size of the signal may be small and again interpretation of the results may be difficult.